The electrical excitability of neuronal membrane was studied using the sucrose gap technique to investigate the sympathetic ganglion of the bullfrog. Action potentials were elicited in three ways: by antidromic stimulation across the sucrose gap; and by orthodromic stimulation through two separate presynaptic pathways. The effects of calcium ions and theophylline were studied on the electrogenic processes involved in action potential generation. We found that the spike after-hyperpolarization in these neurons has a calcium-sensitive component. The calcium sensitive component was found to be an increase in potassium permeability. Furthermore, this calcium-sensitive potassium conductance is potentiated by theophylline, a drug known to affect cellular calcium metabolism. BIBLIOGRAPHIC REFERENCE: Neil A. Busis and Forrest F. Weight, Spike after-hyperpolarization of a sympathetic neuron is calcium-sensitive and is potentiated by theophylline. Nature, 263: 434-436, 1976.